Transmembrane topology of the glutamate receptor subunit GluR6

Ionotropic glutamate receptors mediate most rapid excitatory synaptic transmission in the mammalian central nervous system. These receptors are divided into alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), kainate, and N-methyl-D-aspartate receptors based on pharmacological and electrophysiological characteristics. Ionotropic receptor subunits are integral membrane proteins that have been proposed to have a large extracellular ligand-binding N-terminal domain, four hydrophobic transmembrane domains, and an extracellular C-terminal domain. In this study we have shown that both AMPA receptor subunits (GluR1-4) and kainate receptor subunits (GluR6/7) are glycosylated in adult rat brain; however, the kainate receptor subunits are glycosylated to a greater extent. Examination of the sequences of AMPA and kainate receptors revealed that kainate receptors have several additional consensus sites for N-linked glycosylation; interestingly, one of these is located in the proposed major intracellular loop of the receptor subunits. To test the proposed transmembrane topology model for these receptors, we have used site-specific mutagenesis of the GluR6 subunit to remove the consensus glycosylation site located within the proposed intracellular loop. Mutagenesis of this site demonstrates that it is glycosylated in transiently transfected human embryonic kidney cells, which express functional kainate receptors. Since N-linked glycosylation has only been found to occur on extracellular domains of plasma membrane proteins, these results suggest that the proposed transmembrane topology model for the glutamate receptor subunits is incorrect. Combining these results with other recent data, we have proposed an alternative transmembrane topology model.